A tokamak features a combination of strong toroidal magnetic field, high plasma current and, usually, a large plasma volume and significant auxiliary heating, to provide hot, stable plasma. This allows tokamaks to generate conditions so that fusion can occur. The auxiliary heating (for example via tens of megawatts of neutral beam injection of high energy H, D or T) is necessary to increase the temperature to the sufficiently high values required for nuclear fusion to occur, and/or to maintain the plasma current.
The problem is that, because of the large size, large magnetic fields, and high plasma currents generally required, build costs and running costs are high and the engineering has to be robust to cope with the large stored energies present, both in the magnet systems and in the plasma, which has a risk of ‘disrupting’—mega-ampere currents reducing to zero in a few thousandths of a second in a violent instability.
The situation can be improved by contracting the donut-shaped torus of a conventional tokamak to its limit, having the appearance of a cored apple—the ‘spherical’ tokamak (ST). The first realisation of this concept in the START tokamak at Culham demonstrated a huge increase in efficiency—the magnetic field required to contain a hot plasma can be reduced by a factor of 10. In addition, plasma stability is improved, and building costs reduced.
To obtain the fusion reactions required for economic power generation (i.e. much more power out than power in), a conventional tokamak would have to be huge so that the energy confinement time (which is roughly proportional to plasma volume) can be large enough so that the plasma can be hot enough for thermal fusion to occur.
WO 2013/030554 describes an alternative approach, involving the use of a compact spherical tokamak for use as a neutron source or energy source. The low aspect ratio plasma shape in a spherical tokamak improves the particle confinement time and allows net power generation in a much smaller machine. However, a small diameter central column is a necessity, which presents challenges for design of the plasma confinement vessel and associated magnets.
During the initial phase of starting up a tokamak, the neutral gas which fills the confinement vessel must be ionised to produce a plasma. The process, known as “breakdown”, “formation” or “initiation”, is achieved by passing a time varying current through toroidally wound poloidal field (PF) coils of the tokamak. This time varying current generates a “loop voltage” inside the vessel that, when sufficiently large, causes the gas to break down and form a plasma. The loop voltage produced is a function of the position of the toroidal field coils and the time variation of the current. As well as generating a loop voltage inside the vessel, a current will also be induced in any other toroidally wound conducting loops (e.g. the plasma or the confinement vessel wall).
The most common plasma formation technique uses a solenoid wound in the central column of the tokamak to carry the time varying current and generate the loop voltage. This method is well known, reliable, and used in the majority of tokamaks. However, the compact geometry of spherical tokamaks means that the method is difficult to implement for them—there is limited space in the centre of the torus, and the space is needed for the toroidal field coils, cooling, and neutron shielding. As the size and efficiency of a spherical tokamak is related to the size of the central region, it would be beneficial to operate without a solenoid. Current spherical tokamaks such as MAST and NSTX use a solenoid—but the increased neutron load expected in next generation fusion reactors would make the designs used for those tokamaks impractical due to the extra shielding required.
If a solenoid is not used, then other means must be used to initiate the plasma. Two techniques of interest are “merging compression” (MC) and “double null merging compression” (DNM/double null merging). Plasma formation via merging (either MC or DNM) has further advantages over formation via solenoid. It is theoretically possible (though not yet experimentally confirmed) to achieve high plasma temperature and high plasma currents directly from the merging plasma formation, e.g. plasma temperature greater than 10 keV, which would bring the plasma into the burning regime where self-heating of the plasma is significant. Operating in the burning regime is essential for a practical fusion reactor.
A schematic of the phases of merging compression startup is shown in FIGS. 1A to 1D. Merging compression uses two PF coils 101, one in each of the upper and lower halves of the vacuum vessel 100 to generate the required loop voltage. The current in the PF coils begins at some initial positive value. As this is reduced to zero, two plasma rings 102 are formed around the coils (FIG. 1A). Because each plasma ring carries a current in the same direction, they are attracted towards each other (FIG. 1B) and merge to form a single plasma 103 (FIG. 10). During this merging phase, magnetic energy is converted to kinetic energy in the plasma, accelerating the plasma particles and raising their temperature (“magnetic reconnection”). The speed at which the plasma rings merge can be increased by continuing to reduce the PF coil current below zero, such that it repels the plasma rings and forces them towards each other. The merged plasma is compressed radially inwards, providing additional heating and further increases in the plasma current (FIG. 1D).
Merging compression has been successfully demonstrated on a number of devices, including MAST and START. However, it is impractical for a commercial fusion reactor. The PF coils must be inside the plasma vessel and located close to the final plasma, which means that they would be exposed to intense neutron irradiation and heat flux. The lifetime of such coils would be very limited, and maintaining or replacing components inside the plasma vessel is difficult and expensive.
Double null merging circumvents the problems associated with merging compression by using two pairs of coils, with each pair creating a “null point” in the upper or lower half of the plasma vessel. A “null point” (or X-point”) is a location where the net poloidal magnetic field is zero. The present discussion is concerned only with a null point in the poloidal magnetic field, and there will still be some toroidal magnetic field present at the (PF) null points. As shown in FIG. 2, a PF null point 200 can be formed by passing a current in the same direction through two PF coils 201—the PF null will form at a location between them determined by the relative currents in each coil. As there is no PF in this location, the plasma lifetime is relatively large, so plasma will tend to form around the null points. Once a plasma is generated at the PF null points, it can be merged in a similar manner to MC.
The use of PF coils in pairs to create null points enables these PF coils to be placed outside the plasma vessel. A schematic of DNM is shown in FIGS. 3A to D. In the first phase (FIG. 3A), a PF null 301 is created between each pair of PF coils 302. The nulls are created in the upper and lower halves of the plasma vessel. In the second phase (FIG. 3B), the current in the PF coils is reduced. This generates a loop voltage at the null points, causing the neutral gas to breakdown and form a plasma 303. In the third phase (FIG. 3C) the current continues reducing and becomes negative, repelling the plasma rings from the null points and causing them to merge (undergoing magnetic reconnection 304 as with MC). In the fourth phase (FIG. 3D), the plasma is compressed towards the central column 305 to further increase the plasma temperature and current.
While DNM solves the problem of having to provide coils close to the plasma, it introduces new issues. The PF coils are placed close to the vessel walls—which causes the vessel walls to shield the effects of the changing current in the PF coils. As the current in the PF coils changes, a current is induced in the vessel wall which counteracts the change in current. These “image” or “skin” currents reduce the loop voltage at the null point, making plasma breakdown harder and reducing the induced plasma current. For these reasons, DNM is generally considered impractical for a fusion reactor, especially for a compact reactor such as a spherical tokamak.